Why radar makes a lot of sense for sewage treatment

The water utilities have little experience of radar measurement technology, due partly to the cost of radar instruments. But that could be about to change.

Until recently, level measurement in the water processing and monitoring industries has relied mainly on hydrostatic pressure transmitters and ultrasonic gauges. Thanks to a new generation radar sensor, this is set for change. In many other process industries, radar level technology has become a front-line solution as it delivers more accurate and reliable measurement due to less influence from the process or surrounding environment.
In the UK alone, there are around 9,000 sewage plants, in which about 11B litres of sewage are treated every day. A series of measuring instruments monitor the complex biological processes and form the basis of reliable wastewater treatment. Level gauges, for example, provide information about the quantity of in-flowing wastewater and out-flowing treated water.
These values are needed to control the plant, for example aeration in the activation basins and along with other equipment, they are regularly used for calculating consent charges too. Due to the large amounts of solid material in the sewage, ultrasonic non-contact measuring instruments are often the first choice for this task, measuring over ponds, sumps, weirs and flumes.
Although radar measurement technology is already well-established across many process industries, the water utilities have relatively little experience with this technology. This is due in part to the cost of radar instruments, which in the past were considerably higher than those of ultrasonic, as well as this technology still carrying the image of being complicated.
These preconceptions are now changing.
Jürgen Skowaisa, product manager for radar and ultrasonic at VEGA, considers radar level measurement technology to be a better alternative for the sewage business, primarily because of its robustness in the face of external atmospheric influences. Fog, wind or rain can impair ultrasonic sensors, as well as temperature from solar radiation.
The velocity changes of ultrasonic signals are related to temperature changes (1.6% per 10K). These changes are normally compensated for by a temperature sensor in the transducer.
"As long as it's cloudy, you get reliable values," explains Skowaisa. "But during longer periods of direct sun, temperatures - on the usually black transducers - increase and the sensor temperature becomes much higher than the ambient temperature."
One solution would be installing an additional temperature sensor, but this would mean extra costs and damage risk. Also, surface conditions, such as waves or foam, also affect measuring precision.
The signals are reflected more or less strongly at the surface, depending on how the density and consistency of the foam varies. Compared with ultrasonic, radar measurement technology offers considerable advantages; it operates completely independent of temperature and weather effects.
Even under conditions of strong surface foam generation, latest generation radar sensors bring their benefits to bear through high dynamic sensitivity, i.e. their ability to detect the smallest signals from a water surface beneath foam. For this reason, VEGA is launching VEGAPULS WL61on the market, which is a new radar sensor especially developed for the area of water/sewage.
"It boasts high precision, is very easy to use and with regard to cost is a real alternative to ultrasonic instruments," explains Skowaisa.
"The price differential between ultrasonic and radar measurement technology used to be very high. Today, radar measurement technology is only slightly more expensive, so it is actually the better solution these for these applications."
Some basic information about VEGAPULS WL61: It is a 2-wire, loop-powered device with HART (optional ATEX approval) and an accuracy of +/-2mm, A key feature is the 80mm plastic encapsulated antenna providing excellent focusing, its measuring range extends up to 15m. The instrument has submersible protection rating of IP 66/68 (1 bar), made possible by the moulded cable entry and watertight, robust housing.
A variety of different mounting and communication options make the sensor easy to integrate into existing infrastructures. In the development process, a special emphasis was placed on designing a straightforward, user-friendly operation and handling. Application parameters are thus optimised for the intended areas of use.
"Users in this industry do not need parameters for application in large tanks, storage silos or for solvents as the measured medium," says Skowaisa, "so the new sensor has optimised applications such as pumping station, flow-rate measurement in open channels and other similar applications typical of this sector.
"Linearisation curves can be stored in the sensor itself, so this allows, for example, direct indication of the flow in open channels. The measurement data can be transmitted either directly to a compatible existing control unit or to a SCADA system (4 ... 20 mA, Profibus PA or Foundation Fieldbus), So this makes it a true all rounder."
When it comes to approvals, the user is on the safe side with the new VEGAPULS WL61 as well as ATEX, all existing VEGA radar sensors have been approved according to the prevailing regulations for operation out in the open using microwaves. However, the necessary tests were not always fully defined in the past. To initiate a European guideline that describes the necessary testing scenarios precisely and creates legal certainty for all stakeholders, leading measurement technology companies, including VEGA, got together.
The result is a new European norm with accompanying guidelines in which the test specifications applying to radar level sensors in the future are exactly defined.
The new EN 302729 is to be ratified in the first quarter of 2011. VEGAPULS WL61 is the first measuring instrument on the market that is already in compliance with this norm.
"An important consequence of the new test specifications is that future radar instruments will operate at a frequency of 25GHz instead of the current 26GHz," reports Skowaisa.
But this seemingly tiny modification has had a significant effect on the electronics and antenna, so system designs had to be correspondingly revised.
The following examples show the advantages the new sensor has in daily use:
· Precision measurement: In flow rate measurement in open channels, accuracy is key due to the exponential nature of level to flow calculations. A measurement error of anything up to 10 % can easily arise with ultrasound sensors, for example; the differential of sun on the transducer to the ambient air temperature. Radar technology gives more accurate and reliable measurement data, as it is completely independent of weather effects.
· Stormwater overflows: Stormwater is collected in onsite catchment and stormwater overflow basins to relieve the sewer network and sewage plants during heavy rains. This buffer allows it to be forwarded in a manageable way to the sewage plant after the event. SWO chambers are often sited underground with little headroom.
Here, the user benefits from the small mounting height of the sensor.
Radar technology has almost no blocking distance so measurement can continue even with very high water levels.
· Storage of chemicals: Some chemicals used for sewerage or water treatment and cleaning of processes emit fumes and vapours. They require special handling and measurement so close to the water courses and plants. VEGAPULS radar can be used, even mounted outside, looking through a plastic or GRP vessel to provide a reliable, accurate monitoring for inventory, leakage or overflow.
· Pumping stations: These are needed if the natural descent from households to a sewage plant is not sufficient. The level measurement in the sumps ensures smooth, economical operation of the pumps. Radar technology delivers a convincing performance here as the pump shafts are usually very narrow, and often filled with many pipes, fixtures and fittings and the pumps are sometimes located directly in the shaft.
· Open waterways: Where needed, VEGAPULS WL 61 can be suspended from a cable down into the shaft to reduce interfering reflections. Apart from sewerage and stormwater, another interesting application for radar sensors is the level measurement of waterways, drainage channels, rivers and estuaries.
With its contradictions of scarcity and deluge, the requirement for more monitoring of water levels, supply and sewage treatment will continue to grow in the future, and it will always place new challenges on measurement technology.
Meeting the requirements of the immensely different markets in this sector is in itself a challenge.
In Europe, for example, upgrades for existing plants are mainly in demand, looking for latest technology to automate, while in other countries, such as China and Brazil, new sewage plants with communication systems based on the latest Fieldbus communications are being built almost daily.
The approvals and norms in the individual countries are also quite different.
"Future market and environmental requirements can be quickly met with such a capable core technology," concludes Skowaisa.
"I am convinced that radar instruments will play a major role in achieving this."